Creates a shiny, graphic representation of the hard drive of your choice. Grouping files by category and size makes it easier to clean out the crap that naturally builds up over time. Exposes loads of hidden files that can be cleaned out - get curious and investigate it if you're unsure of what it is that's taking up so much room.
One tip: don't bother deleting sleepimage when you see it. It's tempting, because it doesn't seem to do anything, but it is actually your computer's RAM (and should be about the same size, 1.5-2.5 GB). The CPU stores that information here when the drives go to sleep. Should the battery die, this file will restore your work.
So, you know, don't screw with it too much like some people who have to learn their lessons the hard way.
8/10 - a bit slow in loading and the deletion of files / could be shinier